This invention relates to a method of controlling a water/hydrogen isotopic exchange reaction plant of a single temperature type comprising a combination of an electrolytic system, an exchange reaction column and a recombining system. More particularly, it relates to a control method of such a water/hydrogen isotopic exchange reaction plant for effectively separating heavy water having a concentration lowered by light water (hereinafter referred to as "degraded heavy water") into high concentration heavy water (hereinafter referred to as "product") and high concentration light water (hereinafter referred to as "waste").
When degraded heavy water is separated into product and waste by using a single temperature type water/hydrogen isotopic exchange reaction plant (hereinafter referred to as the "exchange reaction plant"), the resulting product is used as a moderator of a nuclear reactor. Thus, it must hardly contain light water (ordinary, heavy water concentration of at least 99.7 wt %). On the other hand, the resulting waste must hardly contain heavy water (light water concentration of at least 99.9 wt %) to prevent the loss of precious heavy water.
Therefore, the extracting quantities of product and waste from the exchange reaction plant must be in match with the material balance for degraded heavy water, and product and waste suitable for the condition described above cannot be obtained unless control of the extracting quantities is conducted extremely accurately.
Generally, the control described above is conducted by measuring the concentration of heavy water in product, controlling the extracting quantity of product so that the concentration thereof is kept constant and controlling the extracting quantity of waste so that the water quantity inside the exchange reaction plant, which changes with the supply of degraded heavy water and with the extraction of product, is kept constant.
An example of a conventional single temperature type water/hydrogen isotopic exchange reaction plant is shown in FIG. 6 in which reference numeral 1 represents a water/ hydrogen isotopic exchange reaction column (hereinafter referred to as the "reaction column"). The reaction column 1 is divided into two portions 1a and 1b so as to reduce its height. These two portions 1a and 1b, however, function as a unitary reaction column by means of a tank 1c, a pump 1d and a level controller (LC) 1e for the tank. Reference numeral 2 represents an electrolytic system including therein an electrolytic cell having a diaphram, and reference numeral 3 represents a recombining system of hydrogen and oxygen. A level controller (LC) 15 is mounted to the electrolytic system 2.
Hydrogen D and oxygen E generated from the electrolytic system 2 are sent to the bottom of the reaction column 1 and to the recombining system 3, respectively.
Hydrogen introduced into the bottom of the reaction column 1 is exchanged for light hydrogen contained in descending water during its ascent inside the column. Hydrogen leaving the column top of the reaction column 1 is introduced into the recombining system 3, where it is recombined with oxygen introduced into the recombining system 3. Recombined water G is subjected to level control by a level controller (LC) 3a and is introduced into the top of the reaction column 1 as reflux water F by a pump 8.
Degraded heavy water can be separated by the exchange reaction plant described above into product and waste in the following manner. First, degraded heavy water A is introduced into the intermediate portion of the reaction column 1 by a raw material pump 9. Degraded heavy water A then comes into countercurrent contact with heavy hydrogen that is generated from the electrolytic system 2, and is introduced into the bottom of the reaction column 1 to exchange light hydrogen in degraded heavy water A for heavy hydrogen, and flows out from the column bottom as heavy water which is then introduced into the electrolytic system 2.
Hydrogen that comes into contact with degraded heavy water and entraps therein light hydrogen rises further inside the column, comes into countercurrent contact with reflux water F introduced from the column top, exchanges remaining heavy hydrogen for light hydrogen in reflux water F and is introduced as light hydrogen into the recombining system 3. In this instance, reflux water F entraps heavy hydrogen during its descent and attains substantially the same heavy water concentration as that of degraded heavy water A at the portion where this degraded heavy water A is introduced.
The exchange reaction plant operated in the above-mentioned manner is controlled by operating a heavy water concentration controller (ZC) 11 so that the concentration of heavy water in the electrolytic system 2 is kept constant, and operating a control valve 13 through a flow rate controller (FC) 12 to withdraw product B.
On the other hand, waste C is withdrawn by providing a piping extending from the recombining system 3 to the top of the reaction column 1, sending an output of the level controller (LC) 15 of the electrolytic system 2 (which represents the water quantity inside the exchange reaction plant) as a flow rate set signal to a flow rate controller (FC) 16 and operating a control valve 17 by this FC 16.
As a result of the control described above, the concentration of product heavy water and the water quantity inside the exchange reaction plant are kept constant, and the heavy water concentration in product B as well as the light water concentration in waste C have high concentrations as desired.
However, when the water quantity remaining at each portion of the exchange reaction plant is examined, the water retention quantity is small in the reaction column 1, while the water retention quantity in the tank (1c in FIG. 6) (which is provided in the case where the reaction column is divided in order to reduce its height) is extremely great. The residence time of reflux water F ranges from several minutes to several hours. Furthermore, the retention quantity of recombined water G is great in the recombining system 3 but its residence time is also within several hours with respect to the recombined water quantity.
In contrast, since the electrolytic system 2 has an extremely large capacity, the residence time with respect to the extracting quantity of product is about 100 hours.
In accordance with the control method such as shown in FIG. 6, therefore, response to the change of the product concentration is extremely slow and the extracting quantity of waste changes drastically though the heavy water concentration hardly changes. Thus the light water concentration in waste deviates from a predetermined value and the control cannot be made any more.
To cope with this problem, the heavy water concentration in waste can be controlled with high response by extracting waste so that the light water concentration or the heavy water concentraction in waste is kept constant.
Though an extremely expensive infrared absorptiometry is known as a method of measuring continuously a trace amount of heavy water concentration in waste, the application of this method to the control system involves the problems in stability and reliability.